feat: 算力引擎 Go 后端核心（new-api）

- 统一 OpenAI 兼容 /v1 中继 + 渠道/额度/令牌/流水
- 各领域包：relay 模型网关、model 数据层、controller 管理 API

service/return_path_test.go

@@ -0,0 +1,16 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/QuantumNous/new-api/setting/system_setting"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPaymentReturnURLUsesSuppliedDefaultDashboardPath(t *testing.T) {
+	previousAddress := system_setting.ServerAddress
+	system_setting.ServerAddress = "https://dashboard.example.com/"
+	t.Cleanup(func() { system_setting.ServerAddress = previousAddress })
+
+	assert.Equal(t, "https://dashboard.example.com/wallet", PaymentReturnURL("/wallet"))
+}